Hydrogen is generally used to produce ammonia and methanol and is applied to produce saturated compounds as an essential element. Also, it plays a pivotal role in hydrotreating processes, including hydrogen addition, desulfurization, denitrogenation, demetallization and especially the hydrogenation of carbon dioxide, which causes global warning. Furthermore, hydrogen is viewed as a pollution-free energy source and a substitute for existing fossil fuels.
There are many different kinds of conventional methods for producing hydrogen, which include extraction from fossil fuels, such as a naphtha, modification of natural gas, the reaction of vapor with iron at a high temperature, the reaction of water with alkaline metal, electrolysis of water, etc.
However, these techniques are economically unfavorable because immense heat or electric energy is required and, particularly, in the modification of fossil fuels, a large quantity of carbon dioxide is generated as a by-product. With electrolysis, problems, such as the short electrode lifetime and oxygen generated as a by-product, need to be solved. Thus, it has heretofore been economically unfavorable to solve these problems due to the huge cost of facilities for hydrogen production.
Hydrogen gas can readily escape from the gravity of the earth because it is of low specific gravity, and most of it exists in water or inorganic forms. For these reasons, only a small quantity of hydrogen exists in the atmosphere. Also, it is very difficult to purify hydrogen existing in an inorganic form. If possible, it is also not economically favorable. Thus, the development of techniques whereby high-purity hydrogen can be efficiently obtained from water is very important for solving the urgent problem of exploiting substitute energy sources.
Recently, hydrogen producing techniques have been developed in which a photocatalyst is used to decompose water into hydrogen and oxygen. However, there is little published prior art relating to photocatalysts for producing hydrogen. Representative prior art is exemplified by Japanese Pat. Laid-Open Publication Nos. Sho 62-191045 and Sho 63-107815.
Japanese Pat. Laid-Open Publication No. Sho 62-191045 shows that hydrogen is generated from a photolysis reaction of an aqueous Na.sub.2 S solution in the presence of a rare-earth element compound. Also, the rare-earth element compound as a catalyst has an advantage of exhibiting an optical activity in the range of the visible light.
Japanese pat. Laid-Open Publication No. Sho 63-107815 describes a photolysis reaction in which a composite oxide of niobium and alkali earth metal is used as a photocatalyst to generate hydrogen from a methanol solution in water. Likely, this photocatalyst has an advantage of being optically active in the visible light range.
However, the noted prior art is disadvantageous in that the amount of hydrogen generated is small, and the rate of production is 10 ml/0.5g hr.
There are Korean Pat. Appl'n. No. 95-7721, No.95-30416 and No.96-44214, which are believed to solve the above problems.
Korean Pat. Appl'n. No. 95-7721 suggests a photocatalyst represented by the following general formula I: EQU Cs(a)/K.sub.4 Nb.sub.6 O.sub.17 I
In the presence of the photocatalyst of formula I, UV light is irradiated onto an aqueous solution mixed with oxygen-containing organic compounds, such as formaldehyde and alcohol, acting as a hydrogen-generating promoter, to produce hydrogen. This technique has little affect on the environment and can generate hydrogen at room temperature. However, the oxygen-containing organic compounds acting as a hydrogen-generating promoter to produce hydrogen make it impossible to reuse the reactants.
Korean Pat. Appl'n No. 95-30416 suggests a photocatalyst represented by the following formula II: EQU Cs(a)H(c)/S(b) II
This technique also has little affect the environment and can generate hydrogen without the oxygen-containing organic compound as a hydrogen-generating promoter at room temperature but has some problems with the lifetime and stability of the photocatalyst of formula II.
For example, when alkali metal, such as cesium (Cs), is impregnated in a photo-carrier, the amount of hydrogen generated is outstandingly increased but the catalyst stability is decreased.
In addition, Korean Pat. Appl'n No. 96-44214 describes a photocatalyst represented by the following formula III: EQU Pt(a)/Zn[M(b)]S III
This technique likewise has little affect on the environment. Although depending on electron donors and reducing agents, the photocatalyst of formula m is superior in simplicity of preparation, stability, and lifetime, as well as optical activity in the visible light range, compared with previously-noted inventions. But the amount of produced hydrogen is economically unfavorable.